1. Field of the Invention
The present invention relates to a hydrostatic transaxle apparatus, especially, for a vehicle having a vertical crankshaft engine.
2. Background Art
A hydrostatic transaxle apparatus called an IHT comprises a hydrostatic transmission (hereinafter, referred to as an “HST”), an axle and a drive train interposed between the HST and the axle disposed together in a common housing. Some of various IHTs are used for a vehicle having a vertical crankshaft engine, wherein the rotary axis of the hydraulic pump of the HST must be disposed vertically, i.e., in perpendicular to the horizontal axle. If the rotary axis of the hydraulic motor of the HST is disposed horizontally, the drive train between the HST and the axle can be simplified. However, the configuration of a center section for fluidly connecting the hydraulic pump and motor to each other must be complicated. Further, output means such as a motor shaft of the hydraulic motor must be extended horizontally, thereby expanding the IHT in the horizontal direction.
In order to reduce the horizontal size of the IHT for a vertical crankshaft engine, and to simplify the center section of the HST, it is noticed that the rotary axis of the hydraulic motor may be disposed vertically. U.S. Pat. No. 4,979,583 discloses an IHT, wherein a hydraulic pump and a hydraulic motor are disposed vertically oppositely to each other so as to arrange both the rotary axes thereof vertically coaxially to each other.
The hydraulic pump and motor of U.S. patent '583 are of a radial piston type. That is, each of them comprises a rotor serving as a cylinder block, and pistons inserted in the rotor so as to reciprocate radially (horizontally). In the hydraulic pump, a cam ring (track ring) is provided around the rotor. The cam ring is equal to a movable swash plate of an axial piston type hydraulic pump. A track ring is also disposed around the rotor of the hydraulic motor, in place of a swash plate of an axial piston type hydraulic motor. Both the rotors are rotatably provided around a vertically axial common pintle. Oil passages are formed within the pintle so as to constitute a closed circuit between the hydraulic pump and motor. Also, a horizontally plain plate is disposed around the pintle so as to be sandwiched between the hydraulic pump and motor, thereby guiding the cam ring.
The disclosed HST is axially (vertically) short, however, still radially (horizontally) expanded because the track rings must be disposed concentrically with the respective rotors and the pistons must be allowed to reciprocate radially. This is an obstacle to minimization of the IHT in the radial direction of the HST. Further, for positioning the rotors and track rings, the plate to be sandwiched between the hydraulic pump and motor must be provided around the pintle, thereby increasing the number of parts and costs. If the HST was comprised of an axial piston type hydraulic pump and motor, a single member of the name of a center section could be used for fluidal connection of the hydraulic pump and motor and for positioning the hydraulic pump and motor.